Mega‐Tidal and Surface Flooding Controls on Coastal Groundwater and Saltwater Intrusion Within Agricultural Dikelands

Abstract

Climate change will increase sea levels, driving saltwater into coastal aquifers and impacting coastal communities and land use viability. Coastal aquifers are also impacted by tides that control groundwater‐ocean interactions and maintain an “upper saline plume” (USP) of brackish groundwater. Coastal dikes are designed to limit the surface impacts of high‐amplitude tides, but, due to ongoing sea‐level rise (SLR), low‐lying dikelands and underlying aquifers are becoming increasingly vulnerable to flooding from high tides and storm surges. This study combines field observations with numerical modeling to investigate ocean‐aquifer mixing and future saltwater intrusion dynamics in a mega‐tidal (tidal range >8 m) dikeland along the Bay of Fundy in Atlantic Canada. Field data revealed strong connectivity between the ocean and coastal aquifer, as evidenced by pronounced tidal oscillations in deeper groundwater heads and an order of magnitude intra‐tidal change in subsurface electrical resistivity. Numerical model results indicate that SLR and surges will force the migration of the USP landward, amplifying salinization of freshwater resources. Simulated storm surges can overtop the dike, contaminating agricultural soils. The presence of dikes decreased salinization under low surge scenarios, but increased salinization under larger overtopping scenarios due to landward ponding of seawater behind the dike. Mega‐tidal conditions maintain a large USP and impact aquifer freshening rates. Results highlight the vulnerability of terrestrial soil landscapes and freshwater resources to climate change and suggest that the subsurface impacts of dike management decisions should be considered in addition to protection measures associated with surface saltwater intrusion processes.

Saltwater Intrusion Into a Confined Island Aquifer Driven by Erosion, Changing Recharge, Sea‐Level Rise, and Coastal Flooding

“…For example, the insensitivity of the Lennox Island aquifer to coastal storms arises in part from the relatively high ratio of land surface elevation to surge height for the modeled transect. As noted in Section 4.4, other studies for sites with different hydrodynamic and topographic conditions have considered much higher surges and/or lower elevations and shown surges to be important for SWI (e.g., Anderson & Lauer, 2008; LeRoux et al., 2023; Paldor & Michael, 2021). Also, waves and wave runup were not considered in this study as Lennox Island is in a protected bay that does not experience very large waves (Dolan, 2022).…”

“…Until recently, vertical SWI driven by infiltration of flooded seawater following coastal storms had received less attention than SWI driven by SLR (Cantelon et al., 2022). However, recent studies have shown that vertical SWI dynamics in response to coastal flooding depend on many factors including geology, spatiotemporal extent of flooding, inundation frequency, and topography (e.g., LeRoux et al., 2023; Tackley et al., 2023; Vithanage et al., 2012; Yu et al., 2016). Given that changes in coastal storm intensity and frequency are expected to occur with a warmer ocean (Greenan et al., 2019; IPCC, 2021), vertical SWI caused by flooding may occur with shorter return periods that do not provide enough time for aquifer recovery between storms (Paldor & Michael, 2021).…”

Physical and chemical characterization of remote coastal aquifers and submarine groundwater discharge from a glacierized watershed

“…Cross‐shore variability, as shown through a time‐lapse of high and low tides (Figure 2a,b), reveals these zones' dynamic flow and transport behaviour. Very few studies have successfully mapped USPs, even fewer in locations with such a high tidal range (Grünenbaum et al, 2023; LeRoux et al, 2023). To the best of our knowledge, we are the first to delineate a large USP in the GoA driven by these high tidal oscillations.…”

Interrelated Coastal Flooding, Erosion, and Groundwater Salinization on a Barrier Island During Hurricane Fiona

“…Such processes can impact nearshore water quality and disturb biogeochemical processes, ecosystems, and aquaculture operations (Chambers et al, 2013;Rakhimbekova et al, 2023). Results from this unpopulated island suggest that coastal populations along unfortified, sandy coastlines may expect geotechnical hazards (e.g., Paldor, Stark, et al, 2022), infrastructure damage (e.g., Habel et al, 2020;Parkinson, 2021), agricultural losses (e.g., Guimond & Michael, 2021;LeRoux et al, 2023;Tackley et al, 2023), groundwater-driven surface flooding (e.g., Housego et al, 2021), and compromised drinking water supplies (Michael et al, 2017). There is a need to understand how these processes along natural coastlines compare with those along developed coasts, where hardened structures may reduce flooding but consequently slow aquifer recovery (Lee et al, 2019).…”